1. Field of the Invention
The present invention relates to an electronic apparatus including a plurality of ports each used for an interface for supplying power to an external device connected thereto, and a power supply control method.
2. Description of the Related Art
Generally, an electronic apparatus such as a personal computer has various interfaces such that it can be connected to an external device (peripheral device) or its functions can be extended. The interfaces include USB, IEEE 1394, and PS/2, which not only transmit/receive data signals to/from an external device connected thereto via a port but also supply power (power supply voltage) from the main body of the electronic apparatus.
To employ an interface that supplies power to an external device, it is necessary to consider the permissible range of a current value and the like defined by the specifications of the interface. In other words, some measures should be taken to avoid the situations in which an external device consumes power (current) the amount of which is not acceptable as a system and a short circuit occurs between a power supply and a ground for some reason.
Exemplary ones of the measures are as follows:
(1) A circuit arrangement in which a signal line and a power line are connected to an interface port to which an external device is connected. A fuse is provided in the power line as a power supply control circuit to limit an amount of current. If a larger amount of current than a predetermined value flows through the power line, the fuse is blown.
(2) A circuit arrangement for turning on/off a power supply using an FET (field effect transistor) switch that is provided in a power line as a power supply control circuit. If a larger amount of current than a predetermined value flows through the power line, the FET switch turns off in response to a control signal.
In order to control the power supply of two ports, the same number of power supply control circuits as that of the ports are required.
According to the specifications of, e.g., USB, at least 500 mA current needs to be consumed to operate respective USB devices. In the circuits, a power supply is controlled for each of ports based on the premise that the current consumed by an external device connected to each of the ports is about 500 mA. In this case, an external device whose current consumption greatly exceeds 500 mA cannot be used.
The problems of the above-described measures are summarized as follows.
A power supply control circuit such as a fuse and an FET switch for controlling a power supply has to be provided for each port. Accordingly, the total number of circuits increases and so does the packing area.
To particularly support an external device that requires a great amount of power for operation, the number of circuits increases markedly and so does the packing area. The larger the number of types of interface and the number of ports, the much larger the size of the circuit.
In the field of electronic apparatus that requires lightness, thinness, shortness and smallness, it is not desirable to increase the number of power supply control circuits used for interfaces.